The present invention relates to a method of depositing a solid substance on a surface of a substrate in a finely divided deposition. More particularly the present invention relates to such a method in which the substance is dissolved to produce a solution which is in turn dissolved in a liquefied gas and sprayed against the substrate. Upon evaporation of the solvent and liquefied gas the substance is left as the deposit on the substrate.
There are a variety of deposition methods that are particularly adapted to deposit a variety of solids on various substrates. For instance, in soldering operations it is necessary to deposit flux on conductors of electrical components to be soldered on a printed circuit board. In such case, the printed circuit board and conductors can be said to serve as a substrate for the flux. Flux deposition is accomplished by foaming the flux and then brushing the flux containing suspension onto the conductors and conductive areas of the circuit board. This foregoing flux deposition method, however, applies an excessive amount of flux. More recently it has been the practice to dissolve the flux in a solvent and then spray the solution with compressed air onto the conductors and circuit board. Evaporation of the solvent leaves a finely divided deposit of the flux.
The problem of using a solvent to deposit a flux is that many solvents present environmental health hazards. Therefore, there has been a need in the art to replace potentially harmful solvents with environmentally safe compounds. Such a scheme can be found in U.S. Pat. No. 5,106,659 in which paint is thinned or diluted with liquid or supercritical carbon dioxide. The resulting mixture is then sprayed against the surface to be painted.
This paint spraying method of U.S. Pat. No. 5,106,659 is not applicable to all substances. For instance, in case of flux deposition, flux like paint, will not dissolve in carbon dioxide. Attempts to spray a mixture of flux and liquid carbon dioxide have met with failure because a portion of the liquid carbon dioxide, upon discharge from a nozzle into the ambient, expands to produce solid particles. The solid carbon dioxide ice particles act as an abrasive to remove any particles of flux that were initially deposited. Additionally, since flux is not in the finely divided form of paint pigments, flux solids will not be deposited in a uniform freely divided manner if simply mixed with carbon dioxide and sprayed against a substrate surface.
As will be discussed, the present invention relates to a method of depositing a solid substance, such as flux, on a substrate to produce a controlled, freely divided deposit on the substrate.